The invention relates generally to energy measurement, and more specifically to an apparatus and method for performing radiometric measurements of laser or electromagnetic radiation (EMR).
It is often desirable to determine the total energy output and operational parameters for a source of electromagnetic radiation, such as continuous wave, pulse or high power lasers. Various measurement devices have been developed for this purpose, which typically use thermal or photonic detectors.
Thermal detectors operate by absorbing input radiation, which produces a temperature rise on the detector surface and changes some property of the detector, such as resistance, contact potential, or polarization. The simplest form of thermal detection works by intercepting a laser beam with a material of known thermal properties and measuring the absorbed energy in the form of heat. Calorimeters of this sort are indiscriminate in the type of energy they absorb and can be made sensitive enough to measure small changes in temperature for small masses of absorbing material. However, a major disadvantage of this type of meter is the slow response time. Additionally, there must be a cooling period between measurements to allow for the dissipation of residual heat produced in the previous measurement.
A thermopile is a type of thermal detector that can be used to measure thermal radiation. A thermopile is made up of thermocouple junction pairs connected either in series or in parallel. The resulting transducer converts thermal energy directly into electrical energy. Absorption of thermal radiation by one of the thermocouple junctions, called the active junction, increases its temperature. The differential temperature between the active junction and a reference junction, kept at a fixed temperature, produces an electromotive force directly proportional to the differential temperature created. This effect is called a thermoelectric effect.
A thermocouple consists of two different materials which are connected at one end, while the other two ends are attached to a voltage meter. If there is a temperature difference between the common junction and the voltmeter ends, a thermovoltage is shown by the meter. The magnitude of the voltage is a function of the temperature difference, but also dependent on the nature of the two employed materials. In a thermopile, an absorbing material is attached to the active junction and it is placed in the path of an incident radiation source. The absorber collects the incident heat and the absorber, along with the thermocouple active junction, warm up due to the incident radiation. After a short period, the temperature difference between the active junction and the reference junction will stabilize. The thermocouple material in turn converts the temperature difference into a voltage shown by the voltmeter. Thus, the voltmeter reading is a direct measure of the object temperature. This method does not need any mechanics and can accurately sense static signals. Although thermopiles are independent of wavelength, their major disadvantage is a slow response time. Consequently, thermopiles are primarily used to measure slow pulsed or continuous wave laser systems only.
A pyroelectric detector, another type of device for performing radiometric measurements of electromagnetic radiation, is based on the unique properties of asymmetric crystals, which form a surface charge when heated or illuminated by electro-magnetic radiation. As long as the pulsed or chopped incident radiation is slower than the thermal relaxation time of the crystal, the crystal remains in thermal equilibrium and generates a small amount of current from the crystal. If the chopping or pulsing time of the incident laser energy is shorter than the thermal relaxation time of the crystal, then the crystal heats up and causes more current to flow. Pyroelectric detectors can measure laser events as short as a few picoseconds. They are spectrally similar to thermopiles, making them useful for visible and infrared light measurements. These devices, like thermopiles, operate at room temperature, and require some form of amplification of the generated signal. In summary, in order for a thermal detector to capture all of the incident energy, the incident beam is required to terminate upon reaching the surface of a detector, and thus eliminate any further use of the incident beam.
Another class of laser power meters are photonic detectors. This category of detectors can be further grouped into photoconductive, photovoltaic and photoemissive detectors. A photonics detector responds to the number of individual photon incidents onto its active surface. They normally have a relatively high responsivity and fast response time. However, their responsivity is typically wavelength dependent and tends to have very low damage threshold in comparison to the thermal detectors.
Photoconductive detectors that are used for measuring laser beams are made from specifically designed and doped semiconductor materials. When the photonic energy exceeds the valence levels, the semiconductor produces an electron hole, which is swept away into a conduction band. In simple terms, the incident energy of a laser beam increases the conductivity of the material and more current flows. The required energy states (E=hv) can vary with the constituent properties of the materials. Depending upon the semiconductor materials used, these devices can be used to sense radiation at wavelengths of less than one μm or longer infra-red wavelengths of 12-25 μm. Normally, photoconductive semiconductors require cooling for longer wavelengths. Some even require cooling at the shorter (3 μm) wavelengths for increased sensitivity. The speed of these detectors varies as a function of the operating temperature, i.e.; the cooler the detector, the faster the response Photoconductive detectors using silicon have response times as fast as a few picoseconds. If cadmium, germanium, lead and indium based materials are used, these devices typically have a response time measured in microseconds or milliseconds. However, like thermal detectors, photoconductive detectors require the laser beam to impinge upon the semiconductor surface and render the measured pulse unusable after a measurement is made.
Photovoltaic detectors are doped semiconductors and can best be described as a diode with a detector on one side. The diode becomes reverse biased when struck by laser radiation. It conducts current proportionally through a junction and produces a voltage, which is directly related to the amount of incident laser energy or power. Photovoltaic detectors are often called photodiodes. Photovoltaic detectors use some of the same materials as photoconductive detectors, with silicon being the most prevalently used. Silicon photodiodes operate effectively at room temperature (300K), though the best performance can always be achieved by cooling the detector substrate.
Photoemissive detectors operate based on the external photoelectric effect. Photoemisive detectors comprise a surface, typically metal, which releases electrons when struck by photons having an energy value greater than the energy required for an electron to escape the electrostatic barrier presented by the termination of the crystalline material surface. The value of this required energy is known as the work function. Most pure metals have a work function value around 4-5 eV, while other alkali metals have values somewhat lower. If the emitted electron travels through a vacuum with an applied voltage, the device is called a vacuum photodiode. Photoemissive devices can respond to laser energy with wavelengths ranging from 100 nm (UV) to the 1000 nm with higher quantum efficiencies at the shorter wavelengths. Because of the relatively high sensitivities of these devices, and the fact that many electrons are generated for lasers of low energy or operating under continuous wave (CW) mode, the noise generated by the emitted electrons makes it difficult to achieve a wide dynamic range. In addition, as with the aforementioned detector types, the photoemissive detectors rely on the termination of the measured incident radiation.
Most current commercial detectors fall into one of the previously described categories. Therefore, they are limited by one or more of the stated shortcomings, such as low damage threshold, insufficient sensitivity, long response time, long wait time, inability to delivery real-time measurement, or the need for additional parts.
U.S. Pat. No. 4,797,555 to La Mar describes an apparatus that uses a target plate that has a temperature sensitive paint applied on its rear surface. This paint determines the intensity profile of a high energy laser beam. In operation, the front surface of the plate is irradiated by a laser beam. A high speed camera records the isothermal lines formed when the temperature sensitive paint changes from its solid phase to its liquid phase. Isointensity lines are then calculated from the recorded isothermal lines. Although this device provides important data regarding the beam, such as beam profile and intensity, it cannot provide a real-time measurement since the entire laser beam is intercepted by the target plate and transformed into heat. Additionally, cooling time is required between each measurement to dissipate the residual heat from the previous experiment.
U.S. Pat. No. 4,704,030 to Steen teaches a detector to provide in-process beam measurements using a beam deflector set in the path of the beam. An electromechanical transducer is coupled with the deflector to detect mechanical responses of the deflector to an incident beam. One disadvantage of this device is that the accuracy of the detector depends not only on the sensitivity of the electromechanical transducer, but also on its location and the spot size of the beam.
U.S. Pat. No. 4,548,496 discloses a non-destructive laser beam sampling meter whose operation is based on optogalvanic effect, the change in impedance of a gas when exposed to a radiation source such as a laser, in the space between the electrodes of a glow discharge. The device is made to operate on the left side of the Paschen curve, where break down voltage increases with decreasing pressure. The meter is capable of measuring the power of a beam without blocking or unduly perturbing the beam. However, the meter requires a gas chamber which can be filled or evacuated to the desired pressure with an inert working gas. In addition, because the gas chamber is filled and emptied for each measurement, the device has to be precisely recalibrated.
UK Pat. No. 1,127,818 teaches a meter based on the charge effect induced in a piezoelectric crystal illuminated by electromagnetic radiation. However, as with calorimeters, such a device intercepts the entire beam and cannot provide real-time measurements.
U.S. Pat. No. 4,325,252 to Miller et al and U.S. Pat. No. 4,381,148 to Ulrich et al. disclose two methods for non-destructive measurement of laser power by detecting changing gas pressure. Ulrich et al. teaches a device that measures the power of a laser pulse using a gas cell. The gas cell is filled with a radiation-absorbing species that is small enough to allow the laser beam to pass through the cell essentially unaltered. The contraction and expansion of gas within the cell generates acoustic waves which are measured using a microphone. Miller et al. teaches measurement of pressure change within a gas-filled tube as the laser beams passes through. Both designs require a gas cell and a means to pump gas through the cell. In addition, the output voltage of both devices responds to energy density (fluence). Hence, a spot size measurement is required to determine absolute pulse energy. Using ultrasonic transducers or microphones alone to measure the photoacoustic pulse will not provide spot size measurements, so additional instrumentation is required to determine total pulse energy. For example, Ulrich et al. provides an example of a power meter capable of measuring the power of a high energy laser beam using photoacoustic techniques, with minimal destruction of the beam. This power meter includes a cell disposed in the flow path of a gas containing a laser radiation-absorbing species. The absorption coefficient of the absorbing species is small enough to allow the beam to pass though the cell basically unaltered. The concentration of the absorbing species may be varied to modulate its absorption of the laser beam power and produce acoustic waves in the gas, which can be detected and measured to give an absolute measurement of the power in the high energy laser beam. However, because the concentration of radiation-absorbing species needs to be recalibrated for each measurement, it is very difficult to use and cannot provide consecutive real-time measurements. Additionally, Ulrich cannot measure other useful parameters associated with a laser beam, such as spot size or beam profile.